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Date: Wed, 22 Oct 2008 18:44:10 +0200
From: "John RABSON" <john.rabson@wanadoo.fr>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Ferrites - why low frequency limit?
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Jim,

Many thanks for the explanation.

73
John F5VLF

*********** REPLY SEPARATOR  ***********

On 21/10/2008 at 21:40 James Moritz wrote:

>Dear John, LF Group,
>
>There is no strict limit on the frequency range of a particular type of
>ferrite, rather there is an optimum frequency range depending on the
>application.
>
>All ferrites have losses that increase with frequency. At low frequency
>this
>is mainly due to magnetic hysteresis in the core, which results in a loss
>that rises with operating frequency at a given level of magnetic flux, and
>at high frequencies things like eddy currents and dielectric losses
>increase
>in significance too. The lower permeability ferrite materials tend to have
>lower overall loss in the magnetic core material in the LF/MF/HF range, but
>require more turns of wire to achieve a given inductance, or a particular
>maximum level of flux in the core, resulting in higher losses in the
>windings. So there is a trade-off, favouring high permeability materials at
>low frequency where the loss due to hysteresis is relatively low and the
>smaller number of turns needed is a benefit, and low permeability materials
>at higher frequency where fewer turns are required.
>
>Whether a material is suitable at a particular frequency depends a lot on
>what it is being used for. If one looks at the impedance of a particular
>winding, the core losses result in a resistive component that increases
>with
>frequency, and an inductance that is constant at low frequencies, but
>decreases rapidly at high frequencies. At very high frequencies, the
>resistive component may also reduce. So there comes a crossover point when
>the coil impedance becomes mostly resistive, and at higer frequencies still
>the overall impedance of the coil actually reduces (this is ignoring the
>effect of stray capacitance, which will also cause the impedance to reduce
>at frequencies above resonance). The crossover tends to occur at higher
>frequencies for lower permeability materials. For a signal transformer, one
>does not usually care too much about the resistive component, provided the
>overall winding impedance is high, which favours high permeability cores.
>For a high Q coil in a tuned circuit, one wants to minimise the resistive
>component as much as possible, which tends to favour low permeability
>cores.
>For noise supression, one wants to maintain a large impedance over a wide
>frequency range, and a resistive impedance is actually quite useful in
>damping out resonances. Here the upper limit is where the overall impedance
>starts to decrease. For power applications such as SMPSUs and transmitters,
>the trade-off becomes more complicated, because one also must consider flux
>density, temperature rise, size and cost of the core, the effect of a DC
>bias current, etc.
>
>So the reccomended frequency range of a ferrite material is really rather a
>vague notion. Nothing terrible happens at low frequencies, but the windings
>tend to get unmanageably large. At high frequencies, there comes a point
>where the losses are too high for the circuit requirements. But in both
>cases, the frequency limits will depend a lot on what the core is being
>used
>for.
>
>Cheers, Jim Moritz
>73 de M0BMU